This invention generally relates to a reference electrode assembly and its use for monitoring liquids that may have a continuously varying composition, more particularly to a reference electrode assembly having a hydrogen ion-specific electrode that is encapsulated within a polymer matrix, the bulk of said polymer matrix having an open-cell structure, the matrix containing a liquid as the electrode potential causing agent while at its surface the polymer matrix includes a structure preventing an open connection with its environment.
Microelectrode instruments are currently used for continuous in-vivo measurement of physiologically important parameters such as pH and pK. In the application of ion specific, active electrodes of this type, it is typically necessary to have a so-called reference electrode available, with respect to which the potential of the active electrode is measured. Reference electrodes should provide a steady reference potential within the electrochemical circuit. Frequently utilized reference electrodes include Ag/AgCl-electrodes and calomel-electrodes. These types of reference-electrodes have only very limited application for in-vivo use, and they are used primarily in conjunction with in-vitro analyses.
Electrodes of the Ag/AgCl type are extremely sensitive to chlorine ions which are present in varying concentrations in fluid to be monitored such as blood. Also, unprotected Ag/AgCl electrodes exhibit insufficient blood compatibility, evidenced by the fact that, when they are contacted with blood, proteins and platelets from the blood soon adhere to the electrode surface, leading to a changing potential and impedance of the reference electrode with time.
In addition to being limited by having similar insufficient blood compatibility, reference electrodes of the calomel type exhibit a further problem if used in-vivo because they contain a relatively concentrated KCl solution. Apart from difficulties brought about by the continuous diffusion of KCl in the bloodstream by electrodes of this type, there exists the danger that even greater concentrations of KCl could enter the blood stream if ceramic frit or the like that maintains same in place becomes broken or cracked. Because the physiology of the human body cannot tolerate high concentrations of potassium in the bloodstream, attempting an in-vivo use of a calomel reference electrode may be injurious or even fatal.
Heretofore, problems of this type have been controlled by the use of so-called adhesive electrodes, typically of the Ag/AgCl type, which electrodes are not placed in the bloodstream but are adhered to the patient's skin, which is used as the ion conducting path. However, these types of electrodes are disadvantageous because of the relatively long distance between the active electrode and the reference electrode and because of the necessity to include a thermostat for the reference electrode. In addition, these measuring systems are difficult to apply in connection with continuous blood analysis in extra-corporal blood circuits such as those utilized during hemodialysis and open-heart surgical procedures.
Austrian Pat. No. 363,062 describes a reference electrode assembly having an electrode which is encapsulated by a polymer matrix that is a gel prepared from a polyacrylamide, a polymethacrylamide, a polyvinyl alcohol, a polyacrylate, a polymethacrylate, polyvinylpyrrolidone or hydroxy ethyl acetate. Such reference electrode assemblies are described to be utilizable as microelectrodes suitable for direct measurement in living cells, it being possible to incorporate a salt or mixture of salts into its polymer matrix. However, this reference electrode is of the Ag/AgCl-type and is, therefore, not based on an electrode reaction comprising hydrogen ions. Furthermore, the salts to be incorporated into the polymer matrix are neutral salts. Accordingly, such reference electrode assemblies exhibit undesirable sensitivity to pH variations in the liquid to be measured.
It is accordingly a general object of the present invention to provide a reference electrode that is especially suitable for in vivo monitoring or measurement of body fluids, such as blood.
Another object of this invention is to provide a reference electrode and method of using same, which use may be as a disposable reference electrode, and which may include applying same directly to the bloodstream for several days in combination with the active electrode.
Another object of this invention is to provide a reference electrode that is insensitive to pH variations in the measuring liquid during extended or varied time periods.
Another object of the present invention is to provide a reference electrode that incorporates a hydrogen ion-specific electrode.
Another object of this invention is to provide a reference electrode assembly in which the electrode member is within a material that takes part in the equilibrium reaction with the hydrogen ions.